This invention relates to a rotary device, which preferably is a pulley for use with an endless flexible drive element such as a belt, and whose effective diameter is variable in order to achieve a variable transmission ratio between the pulley and another pulley or pulleys with which the flexible drive element cooperates.
The invention has been devised in relation to such a pulley which comprises a pair of sheave members rotatable about an axis and supported for movement relative to one another in the direction along said axis, the sheave members having generally frusto-conical driving surfaces facing one another to define a generally V-section annular groove therebetween; and a drive ring disposed between the sheave members and having an external circumferential surface engagable by the drive belt and side surfaces engaging said facing generally frusto-conical driving surfaces of the sheave members, the drive ring being constrained between the sheave members and being able to assume an eccentric position relative thereto when permitted by the dimensions of the groove defined between the sheave members in accordance with the relative axial position thereof. Such a pulley will hereafter be referred to as a pulley of the kind specified.
In a pulley of the kind specified, as the distance between the driving surfaces of the sheave members increases (i.e. the V-section groove there between becomes wider) the drive ring is able to assume an increasingly eccentric position relative to tie sheave members and is caused to do so by the tension of the belt entrained around it. It may be the tension in the belt which causes the drive ring to assume an eccentric position and increase the distance between the sheave members. As the speed at which the drive ring is driven by the sheave members is dependent on the effective diameter at which they are engaged by the drive ring, the effective diameter of the pulley is decreased.
A pulley of the kind specified may be required to be used in the auxiliary drive system of an internal combustion engine for a motor vehicle. It is usual to arrange for the auxiliaries, which may comprise one or more of the following, namely an alternator, a water pump, a cooling fan, a power steering pump, an air conditioning pump, and so on, to be driven from the engine crank shaft by a belt. If a pulley of the kind specified is provided on the engine crank shaft, a variable transmission ratio may be provided between the crank shaft and the auxiliaries, so that the latter can be driven at a reasonably high speed when the engine is running slowly and yet not be over-speeded when the engine is being operated at its maximum speed. A means for maintaining the required tension in the drive belt or for causing a speed-dependent increase in belt tension, is, of course, additionally required when a pulley of the kind specified is thus used.
When a pulley of the kind specified is in use, it is a requirement that the drive ring should remain in a constant or substantially constant axial position, to maintain its alignment with the other pulley or pulleys with which the drive belt cooperates. It is the object of the present invention to provide a pulley of the kind specified with a convenient construction which enables this requirement to be met. In fulfilling this object, however, it will be appreciated as pointed out hereafter that the present invention is applicable to devices other than a pulley of the kind specified.